(Though it's a lot better idea to use DateTime.TryParse instead of Convert as it lets you tell the user there was a problem instead of your app crashing).
Then you ignore those and try to convert the same user inputs to integer:

if (int.Parse(txtLTime.Text) >int.Parse(txtTime.Text))

One of those two conversions will fail: if the data is a date, the integer conversion will fail; if the user inputs an integer, the DateTime conversions will fail. Either way, your app will crash.

Which tells us nothing - we can't see your screen, so we have no idea what you actually typed in, or what happened when you ran it.
So show us the code fragment, show us what the debugger showed was in the textboxes, and tell us what it did that you didn't expect or didn't do that you did!

So what happened to the "... report problem to user ..." bit? You don't do that, you just return - so if either of the textboxes fails to parse as a valid DateTime then your code does nothing and you don't know about it, or why.